the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Dec 2025
High-Latitude Eddy Statistics from SWOT assessed by in situ observations
Abstract. Mesoscale eddies play a key role in the transport of heat, salt, and momentum, yet their statistical characterization at high latitudes has remained elusive due to the coarse resolution of conventional satellite altimetry. Here we present the first statistical description of mesoscale eddies in the Labrador Sea using observations from the Surface Water and Ocean Topography (SWOT) mission. We apply an eddy-detection algorithm directly to the native 2-km SWOT swaths, without gridding or assimilation, and validate the detections against in situ measurements from shipboard current profiler data from one cruise in 2024, as well as against a statistically derived shipboard current-profiler–based eddy census. The comparison demonstrates excellent agreement in eddy size and intensity, confirming SWOT’s ability to resolve high-latitude mesoscale structures previously undetectable in gridded altimetry. The SWOT-derived eddy census based on a full-calendar year reveals a predominance of energetic anticyclones (Irminger Rings) in the basin interior and smaller cyclones along the continental slopes, with clear seasonal variability linked to boundary current instability. These findings provide the first observational benchmark for mesoscale activity in the Labrador Sea and illustrate SWOT’s potential to extend eddy statistics to high-latitude and ice-influenced regions, opening the way for a global assessment of mesoscale variability.
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Status: open (until 26 Mar 2026)
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RC1: 'Comment on egusphere-2025-6055', Jan Klaus Rieck, 04 Feb 2026
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AC1: 'Reply on general comments from RC1', Charly de Marez, 10 Feb 2026
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We thank the reviewer for their careful reading of the manuscript and for the constructive comments provided. At this stage of the review process, we have received the report from one reviewer and are providing this preliminary response so that both the reviewer and the editor can see how we address these points while the review is still ongoing. Here, we focus on addressing the general comments of RC1, which we consider the most important at this stage. The remaining, more specific comments will be addressed comprehensively once the full set of reviews and the editor’s feedback have been received, in the revised manuscript and final response to reviewers.
General comment #1
We do not intend to suggest that SWOT allows us to observe eddies under sea ice or that previous altimeter were less prone to observe the sea surface height in such region. Rather, our point concerns eddies located in regions that are seasonally ice-covered, i.e. areas that are ice-free during part of the year but where classical altimetry has historically been of limited use.
In these regions, the limitation of conventional gridded altimetry products is not primarily the presence of ice at the time of observation, but the fact that the dominant eddies are too small to be properly resolved by the effective resolution of these products. As a result, mesoscale activity in the seasonal ice zone has remained largely undocumented, even during ice-free periods. SWOT, thanks to its much finer spatial resolution and along-track sampling, allows us to observe these small eddies during the ice-free season in regions that are otherwise considered poorly observable with traditional altimetry. This is the aspect we intended to highlight.
Note that even when the area is ice-free, it remains influenced by sea ice processes. This is where seasonal melting occurs, generating sharp temperature and salinity gradients that are known to favor the development of small-scale, energetic eddies. The observed eddies are therefore linked to the seasonal ice cover, yet remain largely invisible to conventional altimetry.
The text will be revised in both the Introduction and the Discussion to clarify this point and avoid any confusion with the idea of observing eddies beneath sea ice.General comment #2
We agree that the impact of the biharmonic inpainting on eddy detection needed to be quantitatively assessed, and we have performed a sensitivity test using a high-resolution numerical simulation as a controlled reference dataset. Our sensitivity test shows that the inpainting method does not introduce any significant bias in our results for the eddies considered in this study. We refer the reviewer to the attached pdf document including figures from the sensitivity test.
We used outputs from the GIGATL1 simulation (1 km horizontal resolution, 100 vertical levels), which fully resolves the mesoscale dynamics in the subpolar Atlantic, including in regions where the first baroclinic Rossby radius is O(10–15 km) (de Marez et al., 2025). Two representative areas were selected.The method is as follows. The SSH fields (from the simulation) were first interpolated onto a 2-km grid to match SWOT’s effective resolution. From these, we extracted N × 128 domains, which we refer to as the “truth” fields. We then constructed synthetic SWOT-like swaths by introducing gaps corresponding to the nadir region and the swath edges, reproducing the exact data geometry encountered in the real SWOT observations. This produced datasets with the same grid and missing-data structure as those used in our detection method. These gapped fields were subsequently filled using the same biharmonic inpainting procedure as in the manuscript, yielding what we refer to as the “swotlike” fields. Eddy detection was then applied independently to the truth and swotlike fields. This procedure was repeated weekly over one year for the two synthetic passes, allowing a statistical comparison of eddy properties derived from complete versus inpainted data.
The comparison shows that the distributions of eddy amplitude and radius obtained from the truth and swotlike datasets are very similar. For the individual passes where detections do not perfectly coincide, the misdetection rate remains below ~5% over the entire swath. Therefore, over the annual sampling, the statistical distributions of eddy properties are not significantly altered by the inpainting procedure (see histograms in attached document).
A trend nevertheless seems to appear: discrepancies (difference of the number of detected eddies using the two arrays) increase with eddy radius. This behavior is physically expected. Larger eddies have a broader spatial imprint and therefore intersect the gapped regions more frequently, making their reconstruction more sensitive to the inpainting. In contrast, eddies with radii smaller than ~15 km (i.e. diameters smaller than approximately half the swath width) remain mostly constrained by observed data and are only weakly affected by the extrapolation.
These additional diagnostics demonstrate that the biharmonic inpainting does not significantly bias the eddy statistics in the size range primarily analyzed in this study (O(15 km) radius). The method is robust for eddies whose characteristic scale is smaller than half the swath width, which corresponds precisely to the population of eddies that SWOT allows us to observe and that are the focus of this paper.
The manuscript will be revised accordingly. We will in particular mention this sensitivity study with the method section, and emphasize the limitations of the method for eddies with R>15 km.
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AC1: 'Reply on general comments from RC1', Charly de Marez, 10 Feb 2026
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Review of "High-Latitude Eddy Statistics from SWOT assessed by in situ observations" by de Marez et al.
The authors use ungridded, along-track satellite altimetry data from SWOT to detect mesoscale eddies in the Labrador Sea and compare individual eddies and eddy statistics compiled over a full year to eddies detected from in-situ, shipboard ADCP measurements, as well as gridded, lower resolution altimetry data. The study confirms existing knowledge about various types of mesoscale eddies in the Labrador Sea but a longer study period would be necessary to really assess this with confidence. However, the main advancement of the study is the methodolgy of detecting eddies from along-track satellite altimetry, allowing to detect smaller features than from gridded altimetry products. Both, this new detection method and its application to SWOT data make this study a significant contribution to our knowledge of mesoscale eddies.
The manuscript is well structured and written and I only have a few, relatively minor comments.
General comments
Specific Comments
Title: This might be irrelevant as I am not a native english speaker but "assessed by in situ observations" seems to not convey what the authors want to say. I suggest replacing by "compared to in situ observations" or something similar.
l. 2: I suggest adding "in the ocean" after "momentum" in the introductory sentence.
ll. 19-20: I find this slightly misleading as the automatic detection of eddies can be performed on mooring or ADCP data etc. What has been made possible through the gridded products is global detection of eddies.
ll. 23-24: I suggeset rephrasing to "The ability of gridded SLA fields to represent the eddy field has previously been questioned, as they have largely distorted eddy characteristics" unless this distorts the intended meaning.
ll. 33-41: The authors mention two environments where eddies are abundant, they then go on to describe the processes in the MIZ in detail but do not describe anything about the boundary currents. I strongly suggest to add some information on what the eddies do in boundary currents. Also see my General comment 1.
ll. 69-75: It is not clear to me how the reduction of overall energy does not have implications for the presented analysis. I do believe this is true but the authors should rephrase this part and include an explanation of why it is true.
l. 99: If possible, include those datasets in the references.
ll. 120-128: The explanation of the padding and filling is not clear to me from the text alone. I suggest the authors rephrase this in order to avoid that the readers have to switch back between the text and Fig. 1 to understand the process.
ll. 123-128: See General comment 2.
l. 168: Why was the SWOT-detected eddy from June 27 used while the one from June 25 would be closer in time to the ins situ-detected eddy on June 24? Is there an automatic algorithm that decides which of the duplicate detections to keep or is this chosen manually? I suggest the authors describe this process.
ll. 182-183: Are the "shallow eddies" here the ones that are detected in areas where the water depth is < 2000 m? If this is the case I suggest the authors rephrase this, as "shallow eddies detected above depth < 2000 m" gives the impression that the eddies themselves are above 2000 m in the water column.
l. 188: Global datasets of mesoscale eddies (and their statistics) are available from lower resolution altimetry data, e.g. Ioannou et al. (2024), extending to similarly high latitudes. While the higher resolution of SWOT certainly makes the presented statisitcs more valuable, it is not the first time that those statistics are derived.
ll. 198-199: I suggest the authors add a reference that supports their statement that "AEs are less susceptible to steering by background currents and less prone to instability-driven decay".
ll. 204-205: The fact that the eddies are stronger near the slopes might also reflect the fact that those eddies are generated in those regions and are necessarily weaker away from them as they decay.
Fig. 4: I suggest the authors use a plotting algorithm without interpolation such that the 2x1 degree boxes are visually identifiable on the maps.
ll. 209-220: I suspect that the western and eastern regions got mixed up here as the West Greenland current is not in the western region and the eastern region is not the one with the weakest eddies. I suggest the authors check this and also make sure that the western and eastern region are correctly attributed to the columns in Fig. 5.
ll. 214-215: Does the average westward propagation speed of Irminger Rings support the connection between a winter maximum of their generation in the east and the presented spring-autumn maximum in the central region?
l. 227: I suggest removing "(surface-intensified)" as not all the types of eddies mentioned are surface-intensified (convective lenses).
l. 250: Not all datasets used in this study are mentioned here. If the code for the detection from SWOT data is available this should be added here as it is certainly of great interest to the community.
References: In general, I suggest the authors add DOIs to all references.
Minor Comments
l. 25 add a comma after "to be resolved, ..."
l. 34 "sea-ice melt generates"
l. 68: "preserving the balanced"
l. 86: is a duplicate of line 84
l. 99: "Other datasets"
l. 106: some formatting issue with "no—KaRIn—measurements"
l. 115: "MIOST" has not been introduced
l. 149: I suggest replacing "in front of" with "using"
l. 160: "tracks"
l. 232: "detection"
l. 284: Use the final published version.
l. 341: citation has no journal
l. 346: Use the final published version.
References
Ioannou, A., Guez, L., Laxenaire, R., & Speich, S. (2024). Global Assessment of Mesoscale Eddies with TOEddies: Comparison Between Multiple Datasets and Colocation with In Situ Measurements. Remote Sensing, 16(22), 4336. https://doi.org/10.3390/rs16224336